EP1781923A1 - A linear free piston stirling machine - Google Patents

A linear free piston stirling machine

Info

Publication number
EP1781923A1
EP1781923A1 EP05768089A EP05768089A EP1781923A1 EP 1781923 A1 EP1781923 A1 EP 1781923A1 EP 05768089 A EP05768089 A EP 05768089A EP 05768089 A EP05768089 A EP 05768089A EP 1781923 A1 EP1781923 A1 EP 1781923A1
Authority
EP
European Patent Office
Prior art keywords
spring
stiffness
piston
stirling machine
linear free
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP05768089A
Other languages
German (de)
French (fr)
Inventor
Andrew Nicholas Hill
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Microgen Engine Corp Holding BV
Original Assignee
Microgen Energy Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Microgen Energy Ltd filed Critical Microgen Energy Ltd
Publication of EP1781923A1 publication Critical patent/EP1781923A1/en
Withdrawn legal-status Critical Current

Links

Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02G—HOT GAS OR COMBUSTION-PRODUCT POSITIVE-DISPLACEMENT ENGINE PLANTS; USE OF WASTE HEAT OF COMBUSTION ENGINES; NOT OTHERWISE PROVIDED FOR
    • F02G1/00—Hot gas positive-displacement engine plants
    • F02G1/04—Hot gas positive-displacement engine plants of closed-cycle type
    • F02G1/043—Hot gas positive-displacement engine plants of closed-cycle type the engine being operated by expansion and contraction of a mass of working gas which is heated and cooled in one of a plurality of constantly communicating expansible chambers, e.g. Stirling cycle type engines
    • F02G1/0435—Hot gas positive-displacement engine plants of closed-cycle type the engine being operated by expansion and contraction of a mass of working gas which is heated and cooled in one of a plurality of constantly communicating expansible chambers, e.g. Stirling cycle type engines the engine being of the free piston type
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02G—HOT GAS OR COMBUSTION-PRODUCT POSITIVE-DISPLACEMENT ENGINE PLANTS; USE OF WASTE HEAT OF COMBUSTION ENGINES; NOT OTHERWISE PROVIDED FOR
    • F02G1/00—Hot gas positive-displacement engine plants
    • F02G1/04—Hot gas positive-displacement engine plants of closed-cycle type
    • F02G1/043—Hot gas positive-displacement engine plants of closed-cycle type the engine being operated by expansion and contraction of a mass of working gas which is heated and cooled in one of a plurality of constantly communicating expansible chambers, e.g. Stirling cycle type engines
    • F02G1/053—Component parts or details

Definitions

  • the present invention relates to a linear free piston Stirling machine, such as an engine or cooler.
  • a linear free piston Stirling engine typically comprises a displacer piston and a power piston each of which reciprocates independently within the engine as is well known in the art .
  • the displacer piston has a flexible rod which extends through the power piston and is then mounted on a pair of plate springs.
  • the displacer piston mass and spring stiffness coupled by the flexible rod cause the displacer piston to move up and down at resonance.
  • the present invention is directed to providing an alternative method of preventing over-stroking.
  • a linear free piston Stirling machine comprising a displacer piston and a power piston, the displacer piston being reciprocally mounted on first and second planar springs, wherein the first spring has a stiffness which increases with increasing spring displacement and the second piston has a stiffness which, in relation to the first spring, is less variable with a spring displacement.
  • the combined response of the springs is such that, within the normal operating range of the engine, the second spring influences the displacer piston to a relatively greater degree, while the first spring has a relatively greater influence outside of the normal operating range.
  • the pair of springs can be designed to fully satisfy the requirement to have a specific resonance stiffness during normal operating conditions which reduces the energy wasted, and a higher stiffness as the stroke limit is reached. This would be difficult and costly to achieve using two identical springs.
  • each planar spring has a plurality of spiral spring portions and a stress relieving hole positioned beyond the outer end of each spiral spring portion. This hole is positioned in the load path in the radially outer ring portion of the planar spring.
  • each stress relieving hole is sized and positioned to provide the required stiffness characteristics.
  • This feature forms a second aspect of the present invention which is broadly defined as a linear free piston Stirling machine comprising a displacer piston and a power piston, the displacer piston being reciprocally mounted on first and second planar springs, wherein at least one of the springs has a plurality of spiral spring portions and a stress relieving hole positioned beyond the outer end of each spiral spring portion.
  • This invention may be used independently of or in conjunction with the first aspect of the invention.
  • Fig. 1 is a schematic cross section showing a Stirling engine
  • Fig. 2 is a perspective view of one of the springs
  • Fig. 3 is a graph showing the variation of the stiffness of the springs with displacement.
  • Fig. 4 is a graph showing the variation in spring stiffness for hole position with different hole diameters.
  • FIG. 1 A linear free piston Stirling engine is shown schematically in Fig. 1.
  • the basic design of the engine is well known in the art (for example see page 9, Figure 2, "Free-Piston Stirling Design Features", Lane, N.W. and Beale, W.T.; 1997 [Review of current design features of free-piston Stirling engines of 3.0 and 1.1 kW output.] , available at www.Sunpower.com/technology. Presented at the Eight International Stirling Engine Conference, May 27-3oth 1997, University of Ancona, Italy) .
  • the engine has a head 1 having fins 2 which are heated by a burner (not shown) .
  • a displacer piston 3 Within the engine housing, are a displacer piston 3 and a power piston 4 which reciprocate relatively to one another.
  • the power piston 4 cooperates with an alternator 5 to generate electrical power.
  • the displacer piston 3 has a flexible rod 6 which extends through the centre of the power piston 4, which is mounted on a pair of planar springs 7. These are bolted by bolts 8 to the engine housing. As the displacer piston 3 reciprocates the planar springs 7 flex thereby creating a restoring force on the displacer piston to return it to its equilibrium position.
  • the spring 7 has a flat circular configuration and can be stamped from sheet metal, polished, drilled and tempered.
  • the spring has a pair of spiral cut-outs 9 which are symmetrical about an axis. Each of the spiral cut-outs 9 terminates at its radially outermost end with a radiused cut-out 10.
  • a pair of mounting holes 11 are provided on opposite sides of the spring to receive the bolts 8.
  • a central hole 12 receives the flexible rod 6 of the displacer piston 3.
  • the spiral cut-outs 9 form a pair of spiral spring portions 13. These have a generally constant cross-section, but become slightly wider at their radially outermost portions 14. Beyond the end of the spiral spring portions 13 are stress relieving holes 15.
  • the combination of the increased cross-section 14 of the radially outermost portions of the spiral spring portions and the stress relieving holes 15 provides a spring design with increased reliability. Effectively, the stress relieving holes 15 serve to minimise the peak stresses at the radially outermost end of the spiral spring portions 13 and transfer these stresses more smoothly across the remainder of the spring material radially outwardly of the spiral spring portions.
  • the characteristics of the spring in terms of its stiffness for a given displacement can be varied simply by changing the size of the stress relieving hole or its precise position with respect to the outer extremity of the spiral spring portion.
  • the two springs 7 are designed to have different stiffness characteristics for variations in displacement as shown in Fig. 3.
  • Line 20 indicates the upper limit of desired displacement for the displacer piston 3. Below this limit, it is desirable to keep the stiffness of the spring pair at a constant level which is just sufficient to provide the required restoring force. Above this limit, it is desired to raise the stiffness as quickly as is practical in order to prevent over-travel of the displacer.
  • the first spring has a stiffness designated by line 21 which is initially low at low displacement, but which rises, initially slowly towards the limit 20. The rise becomes steeper towards the limit 20, and then steeper again beyond the limit.
  • the second spring stiffness designated by line 22, remains substantially constant for all displacements. The stiffness of the second spring initially starts out greater than the stiffness of the first spring, but the stiffness of the first spring exceeds the stiffness of the second spring at around the limit.
  • the combined stiffness of the two springs is shown in Fig. 3 as line 23. It can be seen that, below the limit, and for the normal range of travel of the displacer piston 3, the stiffness is generally constant, although increases slightly. Towards the end of the limit the stiffness begins to rise sharply, and this carries on beyond the limit. Effectively, the combined stiffness is dominated in the lower displacement region by the second spring and in the higher displacement regions by the first spring.
  • the combined characteristic may be achieved by combinations of springs different from those shown in Fig. 3.
  • a second spring is shown substantially constant, it could also increase gradually, while the response of the first spring could be adjusted accordingly. With such an arrangement, it may not be necessary for the stiffness of the second spring to exceed the stiffness of the first spring in the regions of low displacement .
  • the position of the stress relieving holes is not something which is calculated but is rather determined by trial and error.
  • a finite element analysis model was used so as to determine the position of the stress relieving holes which maintain as equal a stress distribution around the hole as possible.
  • Original designs were modelled and found to have high stress concentrations across the inner end of the spring portions 13, and indeed this is where fractures occur during operation.
  • the optimised design was obtained iteratively, using finite element analysis.
  • Figure 4 provides some indication of how the stiffness of the spring varies with the position and size of the hole.
  • the hole position is defined with reference to a line extending from position X to position Y as shown in Figure 2. It can be seen that as the hole is moved towards position Y, the spring stiffness increases. Also, the spring stiffness is seen to decrease with increasing hole diameter.
  • the shaded areas on either side of the graph indicate the limit of this design as, when the hole diameter increases, it is clearly not possible to move fully from X to Y.

Landscapes

  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Combustion & Propulsion (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Springs (AREA)

Abstract

A linear free piston Stirling machine having a displacer piston (3) mounted on first and second planar springs (7). The first spring has a stiffness which increases with increasing spring displacement. The second has a stiffness which is less variable with spring displacement to provide a combined stiffness which rapidly increases once the displacer piston displacement exceeds a fixed limit. Each of the springs (7) has a plurality of spiral spring portions (13) and a stress relieving hole (15) beyond the outer edge of the spiral spring portions.

Description

A LINEAR FREE PISTON STIRLING MACHINE
The present invention relates to a linear free piston Stirling machine, such as an engine or cooler.
A linear free piston Stirling engine typically comprises a displacer piston and a power piston each of which reciprocates independently within the engine as is well known in the art .
In one construction, the displacer piston has a flexible rod which extends through the power piston and is then mounted on a pair of plate springs. The displacer piston mass and spring stiffness coupled by the flexible rod cause the displacer piston to move up and down at resonance.
In order to avoid collision noise and overstressing of the planar springs, it is necessary to maintain the amplitude of reciprocation of the displacer piston within certain physical limits of the design.
A number of ways have been proposed to overcome this problem including the use of spring magnets on the power piston to prevent over-stroking as shown in US4, 937,481. However, these spring magnets produce fringing fields which interact with the magnetic flux from the main magnets and reduce the engine efficiency.
The present invention is directed to providing an alternative method of preventing over-stroking. According to the present invention there is provided a linear free piston Stirling machine comprising a displacer piston and a power piston, the displacer piston being reciprocally mounted on first and second planar springs, wherein the first spring has a stiffness which increases with increasing spring displacement and the second piston has a stiffness which, in relation to the first spring, is less variable with a spring displacement.
By configuring the springs in this way, the combined response of the springs is such that, within the normal operating range of the engine, the second spring influences the displacer piston to a relatively greater degree, while the first spring has a relatively greater influence outside of the normal operating range. Thus, the pair of springs can be designed to fully satisfy the requirement to have a specific resonance stiffness during normal operating conditions which reduces the energy wasted, and a higher stiffness as the stroke limit is reached. This would be difficult and costly to achieve using two identical springs.
Preferably each planar spring has a plurality of spiral spring portions and a stress relieving hole positioned beyond the outer end of each spiral spring portion. This hole is positioned in the load path in the radially outer ring portion of the planar spring.
Preferably each stress relieving hole is sized and positioned to provide the required stiffness characteristics. This feature forms a second aspect of the present invention which is broadly defined as a linear free piston Stirling machine comprising a displacer piston and a power piston, the displacer piston being reciprocally mounted on first and second planar springs, wherein at least one of the springs has a plurality of spiral spring portions and a stress relieving hole positioned beyond the outer end of each spiral spring portion. This invention may be used independently of or in conjunction with the first aspect of the invention.
An example of a linear free piston Stirling machine in accordance with the present invention will now be described with reference to the accompanying drawings, in which:
Fig. 1 is a schematic cross section showing a Stirling engine;
Fig. 2 is a perspective view of one of the springs;
Fig. 3 is a graph showing the variation of the stiffness of the springs with displacement; and
Fig. 4 is a graph showing the variation in spring stiffness for hole position with different hole diameters.
A linear free piston Stirling engine is shown schematically in Fig. 1. The basic design of the engine is well known in the art (for example see page 9, Figure 2, "Free-Piston Stirling Design Features", Lane, N.W. and Beale, W.T.; 1997 [Review of current design features of free-piston Stirling engines of 3.0 and 1.1 kW output.] , available at www.Sunpower.com/technology. Presented at the Eight International Stirling Engine Conference, May 27-3oth 1997, University of Ancona, Italy) .
In simple terms, the engine has a head 1 having fins 2 which are heated by a burner (not shown) . Within the engine housing, are a displacer piston 3 and a power piston 4 which reciprocate relatively to one another. The power piston 4 cooperates with an alternator 5 to generate electrical power.
The displacer piston 3 has a flexible rod 6 which extends through the centre of the power piston 4, which is mounted on a pair of planar springs 7. These are bolted by bolts 8 to the engine housing. As the displacer piston 3 reciprocates the planar springs 7 flex thereby creating a restoring force on the displacer piston to return it to its equilibrium position.
The design of one of the springs 7 is shown in greater detail in Fig. 2. As can be seen, the spring 7 has a flat circular configuration and can be stamped from sheet metal, polished, drilled and tempered. The spring has a pair of spiral cut-outs 9 which are symmetrical about an axis. Each of the spiral cut-outs 9 terminates at its radially outermost end with a radiused cut-out 10. A pair of mounting holes 11 are provided on opposite sides of the spring to receive the bolts 8. A central hole 12 receives the flexible rod 6 of the displacer piston 3.
The spiral cut-outs 9 form a pair of spiral spring portions 13. These have a generally constant cross-section, but become slightly wider at their radially outermost portions 14. Beyond the end of the spiral spring portions 13 are stress relieving holes 15.
The combination of the increased cross-section 14 of the radially outermost portions of the spiral spring portions and the stress relieving holes 15 provides a spring design with increased reliability. Effectively, the stress relieving holes 15 serve to minimise the peak stresses at the radially outermost end of the spiral spring portions 13 and transfer these stresses more smoothly across the remainder of the spring material radially outwardly of the spiral spring portions.
The characteristics of the spring in terms of its stiffness for a given displacement can be varied simply by changing the size of the stress relieving hole or its precise position with respect to the outer extremity of the spiral spring portion.
The two springs 7 are designed to have different stiffness characteristics for variations in displacement as shown in Fig. 3. Line 20 indicates the upper limit of desired displacement for the displacer piston 3. Below this limit, it is desirable to keep the stiffness of the spring pair at a constant level which is just sufficient to provide the required restoring force. Above this limit, it is desired to raise the stiffness as quickly as is practical in order to prevent over-travel of the displacer.
As can be seen from Fig. 3, the first spring has a stiffness designated by line 21 which is initially low at low displacement, but which rises, initially slowly towards the limit 20. The rise becomes steeper towards the limit 20, and then steeper again beyond the limit. By contrast, the second spring stiffness, designated by line 22, remains substantially constant for all displacements. The stiffness of the second spring initially starts out greater than the stiffness of the first spring, but the stiffness of the first spring exceeds the stiffness of the second spring at around the limit.
The combined stiffness of the two springs is shown in Fig. 3 as line 23. It can be seen that, below the limit, and for the normal range of travel of the displacer piston 3, the stiffness is generally constant, although increases slightly. Towards the end of the limit the stiffness begins to rise sharply, and this carries on beyond the limit. Effectively, the combined stiffness is dominated in the lower displacement region by the second spring and in the higher displacement regions by the first spring.
It should be appreciated that the combined characteristic may be achieved by combinations of springs different from those shown in Fig. 3. Thus, for example, although a second spring is shown substantially constant, it could also increase gradually, while the response of the first spring could be adjusted accordingly. With such an arrangement, it may not be necessary for the stiffness of the second spring to exceed the stiffness of the first spring in the regions of low displacement .
The position of the stress relieving holes is not something which is calculated but is rather determined by trial and error. A finite element analysis model was used so as to determine the position of the stress relieving holes which maintain as equal a stress distribution around the hole as possible. Original designs were modelled and found to have high stress concentrations across the inner end of the spring portions 13, and indeed this is where fractures occur during operation. The optimised design was obtained iteratively, using finite element analysis. Figure 4 provides some indication of how the stiffness of the spring varies with the position and size of the hole. The hole position is defined with reference to a line extending from position X to position Y as shown in Figure 2. It can be seen that as the hole is moved towards position Y, the spring stiffness increases. Also, the spring stiffness is seen to decrease with increasing hole diameter. The shaded areas on either side of the graph indicate the limit of this design as, when the hole diameter increases, it is clearly not possible to move fully from X to Y.

Claims

Claims
1. A linear free piston Stirling machine comprising a displacer piston and a power piston, the displacer piston being reciprocally mounted on first and second planar springs, wherein the first spring has a stiffness which increases with increasing spring displacement and the second spring has a stiffness which, in relation to the first spring, is less variable with spring displacement.
2. A linear free piston Stirling machine according to claim 1, wherein each planar spring has a plurality of spiral spring portions and a stress relieving hole positioned beyond the outer end of each spiral spring portion.
3. A linear free piston Stirling machine according to claim 2, wherein the spiral spring portions increase in width towards their radially outermost end.
4. A linear free piston Stirling machine according to claim 2 or claim 3, wherein each stress relieving hole is sized to provide the required stiffness characteristics.
5. A linear free piston Stirling machine comprising a displacer piston and a power piston, the displacer piston being reciprocally mounted on first and second planar springs, wherein at least one of the springs has a plurality of spiral spring portions and a stress relieving hole positioned beyond the outer end of each spiral spring portion.
6. A linear free piston Stirling machine according to claim 5, wherein the spiral spring portions increase in width towards their radially outermost end.
653180; MJD; CAR
EP05768089A 2004-08-06 2005-08-04 A linear free piston stirling machine Withdrawn EP1781923A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GBGB0417611.1A GB0417611D0 (en) 2004-08-06 2004-08-06 A linear free piston stirling machine
PCT/GB2005/003066 WO2006013377A1 (en) 2004-08-06 2005-08-04 A linear free piston stirling machine

Publications (1)

Publication Number Publication Date
EP1781923A1 true EP1781923A1 (en) 2007-05-09

Family

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Family Applications (1)

Application Number Title Priority Date Filing Date
EP05768089A Withdrawn EP1781923A1 (en) 2004-08-06 2005-08-04 A linear free piston stirling machine

Country Status (5)

Country Link
US (1) US20090094977A1 (en)
EP (1) EP1781923A1 (en)
JP (1) JP2008509321A (en)
GB (1) GB0417611D0 (en)
WO (1) WO2006013377A1 (en)

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US8590301B2 (en) * 2010-12-14 2013-11-26 Sunpower, Inc. Free-piston stirling machine for extreme temperatures
US8752375B2 (en) * 2011-08-16 2014-06-17 Global Cooling, Inc. Free-piston stirling machine in an opposed piston gamma configuration having improved stability, efficiency and control
JP5754642B2 (en) 2011-09-16 2015-07-29 いすゞ自動車株式会社 Free piston type Stirling engine
CN104514828B (en) 2013-09-30 2016-06-08 珠海格力节能环保制冷技术研究中心有限公司 Flat spring and plate spring groups and compressor
US10036370B2 (en) 2014-02-10 2018-07-31 Haier Us Appliance Solutions, Inc. Linear compressor
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US9562525B2 (en) * 2014-02-10 2017-02-07 Haier Us Appliance Solutions, Inc. Linear compressor
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US10208741B2 (en) 2015-01-28 2019-02-19 Haier Us Appliance Solutions, Inc. Method for operating a linear compressor
US10502201B2 (en) 2015-01-28 2019-12-10 Haier Us Appliance Solutions, Inc. Method for operating a linear compressor
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US10830230B2 (en) 2017-01-04 2020-11-10 Haier Us Appliance Solutions, Inc. Method for operating a linear compressor
US10641263B2 (en) 2017-08-31 2020-05-05 Haier Us Appliance Solutions, Inc. Method for operating a linear compressor
US10670008B2 (en) 2017-08-31 2020-06-02 Haier Us Appliance Solutions, Inc. Method for detecting head crashing in a linear compressor
US11209192B2 (en) * 2019-07-29 2021-12-28 Cryo Tech Ltd. Cryogenic Stirling refrigerator with a pneumatic expander
CN112157656B (en) * 2020-09-23 2022-03-08 哈尔滨工业大学 Symmetrical variable fulcrum rigidity adjusting module capable of realizing full-range rigidity adjustment
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Also Published As

Publication number Publication date
WO2006013377A1 (en) 2006-02-09
GB0417611D0 (en) 2004-09-08
JP2008509321A (en) 2008-03-27
US20090094977A1 (en) 2009-04-16

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